New (old stock) MRRC AC Cobra really noisy and shaky

2843 Views 19 Replies 13 Participants Last post by Abarth Mike, May 17, 2021

EdGasket

Discussion Starter · Mar 14, 2021 (Edited by Moderator)

Hi, I recently purchased a 'New, old stock' MRRC cobra model 9911 number 10 with a silver/white motor. I guess this has been shelved for some 20 years or so. There were the inevitable flat spots on the tyres where the car was clamped down to the box however the main problem is that the car runs with a lot of noise, like catching/clicking. On running the chassis only it looks like the motor and driveshaft are moving from side to side and also throwing the back axle left and right. The spring connecting the motor to the driveshaft does not look entirely straight, rather like it has a slight bend in it. Could that spring have got out of true from long storage in the same position?

Anyway this is my first experience with front mounted motors and driveshafts. Is this likely to break in and calm down or is there a problem I need to sort out, like try a replacement spring or something else? Do the motors and driveshafts pop out of the plastic OK on these old models or is there a risk of snapping the chassis?

Any advice on how to improve the running would be appreciated.

I also notice I can use full power round the circuit without the car de-slotting (it has a magnet so goes reasonably fast but not as fast as some of my other cars on much less power) but after a few laps the motor is very hot. Is this due to the misalignment of the running gear wasting power do you think?

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matthew77

· #2 · Mar 14, 2021

The Cobra is a quick car but possibly struggling for top speed. If you can, try to straighten the spring as it may be making for rough running, causing the motor to struggle. Also check the gears are meshing ok.

It is ok to pop the driveshaft out without breaking the chassis. When you are happy to refit it, use a small amount of glue on the driveshaft bearing so it doesnt pop out in use.

Matthew

stuckinthe60s

· #3 · Mar 14, 2021

the car being installed on it's mounting plate too tightly, and then sitting for many years, could not only flatten tires, but pop the components out of their mounting clips inside, causing the "bent" spring and other alignment issues in the drivetrain. Can't tell if you've already had it apart, but do so and check these things. While you are there, do as Matthew suggests with the bearing, then lube all bearing surfaces with light oil, including the motor shaft.

Steve

Mr Modifier

· #4 · Mar 14, 2021

If the motor is very hot but not pushing the car forwards very fast the motor is having to work too hard. This will be due to friction in the drive train. This could be a sticking bush or a loose bush rotating in its mount, but as you mention noise I would investigate gear mesh. Too tight a mesh and the gears would bind and you will notice this when trying to rotate the rear wheels by hand. Poor mesh can also be a problem as this will mean the gears could be slipping (producing noise) and they will quickly wear out and fail. Loose mesh is most common (in my experience) with in-line setups. If your axle moves left and right then the gears wont be meshing - especially when cornering forces move the contrate away from the pinion. An axle stopper will help.

A tiny amount of lubricant is essential on gears - a light grease will hand on to the gears better than a light oil.

Have. A look at the teeth on the pinion and the contrate - if they show any sign of wear or the teeth are asymmetric - you have a mesh issue.

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EdGasket

Discussion Starter · #5 · Mar 15, 2021

stuckinthe60s said:
the car being installed on it's mounting plate too tightly, and then sitting for many years, could not only flatten tires, but pop the components out of their mounting clips inside, causing the "bent" spring and other alignment issues in the drivetrain. Can't tell if you've already had it apart, but do so and check these things. While you are there, do as Matthew suggests with the bearing, then lube all bearing surfaces with light oil, including the motor shaft.

Steve

I think the car being clamped to the box base may have flexed the chassis downwards and hence pointed both the motor and driveshaft down and the spring has 'aged' with a bend in it. Now it just throws the motor, shaft and back axle around. Do you know anywhere in the UK that sells replacement coupling springs or any substitute that will work?

Mr.M

· #6 · Mar 15, 2021

If you are happy that the motor and shaft are now reasonably well aligned, I have read on here of people using the inner of a Biro pen cut to the required length. Apparently it has enough strength/flexibility to replace the spring, which I think are only available with a replacement motor.

Welcome to the forum.

matthew77

· #7 · Mar 16, 2021

I did see the motor springs available some years ago. Sadly I dont know if they are still available. I saw them in a list of Fly spares at Gaugemaster.

The biro pen innards sounds a brilliant solution and 1 I hope to try soon once my track is rebuilt.

zendragon

· #8 · Mar 16, 2021

Sir I am sorry I dont know of an AC Cobra that had a drive shaft to the rear wheels, (not to say there wasnt one), but fly used to have a front mounted motor driving the rear wheels with a shaft so maybe you could get a shaft or spring from pendle.

As an aside if you have made sure that the shaft is clipped into the chassis and the motor and rear axle are the same then they shouldnt vibrate, I would recommend that you flatten the chassis (baking tray and magnets in moderate oven (others will advise temperature)), and put a spot of super glue under the bearings use plenty of oil around the bearings when glueing them.

Will watch with interest

Zen

42redcat

· #9 · Mar 16, 2021

I entered 'slot car driveshaft' on the online auction site and turned up a new old stock MRRC Cobra driveshaft for £4.99 +. £1.50 postage. Might be worth a look.

Mike

j-c

· #10 · Mar 16, 2021 (Edited by Moderator)

Mr.M said:
.....I have read on here of people using the inner of a Biro pen cut to the required length......

Do you know what colour Biro is best?

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Mr.M

· #11 · Mar 16, 2021

Black for wet weather.

Blue for dry.

Red for ultimate speed.

And, if you can find one, green for offroad.....

Reactions: 3

corsa

· #12 · Mar 21, 2021

EdGasket said:
Hi, I recently purchased a 'New, old stock' MRRC cobra model 9911 number 10 with a silver/white motor. I guess this has been shelved for some 20 years or so. There were the inevitable flat spots on the tyres where the car was clamped down to the box however the main problem is that the car runs with a lot of noise, like catching/clicking. On running the chassis only it looks like the motor and driveshaft are moving from side to side and also throwing the back axle left and right. The spring connecting the motor to the driveshaft does not look entirely straight, rather like it has a slight bend in it. Could that spring have got out of true from long storage in the same position?

Anyway this is my first experience with front mounted motors and driveshafts. Is this likely to break in and calm down or is there a problem I need to sort out, like try a replacement spring or something else? Do the motors and driveshafts pop out of the plastic OK on these old models or is there a risk of snapping the chassis?

Any advice on how to improve the running would be appreciated.

I also notice I can use full power round the circuit without the car de-slotting (it has a magnet so goes reasonably fast but not as fast as some of my other cars on much less power) but after a few laps the motor is very hot. Is this due to the misalignment of the running gear wasting power do you think?

As it happens, I have just rescued my 9911 #10. I bought it second hand many years ago and it never went well, and found itself on a shelf. During the winter of lockdown, I have been getting my 20 year old Fly cars back to life and the Cobra came out of the display case as well. The nylon pinion was split, hence poor mesh and noise. Obviously, motor had to come out to replace the pinion, there's no problem, but it is always a bit scary stressing old plastic, and it went back in fine. I removed the rear axle to try with a slot.it rear end and then went back to the original which was quieter and quicker. The bearings pop in and out without issue. The original tyres were glued on, but after removing the gunk, I fitted some NSR 5209s to the original wheels, with some axle spacers and it goes really well with the magnet on my Policar track.

Reactions: 1

Zippideedoda

· #13 · Apr 16, 2021

I bought a MRRC Cobra used on a well known auction site. When I received it found it was a noisy runner and if stopped at the right spot would not get going again without a push. As it turned out, and was mentioned on another forum, the white plastic pinion gear was cracked and no longer meshing properly with the crown gear. It was difficult to see without a magnifying glass. I replaced it with a Slot.it brass pinion but bent the driveshaft spring in the process. Luckily I saw a video where they simply replaced this spring with a very small diameter plastic tube that fit snugly on the driveshafts.

Now the car runs much quieter and without the stopping that occurred when the cracked pinion gear froze up on the crown gear. Hope this helps.

PetesLightKits

· #14 · Apr 16, 2021

Mr.M said:
If you are happy that the motor and shaft are now reasonably well aligned, I have read on here of people using the inner of a Biro pen cut to the required length. Apparently it has enough strength/flexibility to replace the spring, which I think are only available with a replacement motor.

Welcome to the forum.

You can also use a short section of 18 gauge wire insulation (normal everyday household appliance or speaker wire). Cut a piece about 1/2" longer than required, then strip that 1/2" off, then grab the copper wire and pull it out of the remaining section of insulation. Then press the insulation onto the motor shaft and shaft extension. Reassemble the car and away you go!

Peter

EdGasket

Discussion Starter · #15 · May 16, 2021

Finally got back to looking at this; sorry for the delay, other things came up.

I have managed to improve the noise quite considerably though it is still noisy compared to my cars with no extended driveshaft. Here is what I did:

1) Removed all components from the chassis

2) Warmed chassis with a hair dryer and bent it back to a more flat shape. Looks like where it had been heavily clamped down in the box for years, the flexible chassis had bowed.

3) Straightened the spring/driveshaft as best I could by bending the spring back the other way so that it was as best in line with the motor as I could reasonably get.

4) Reassembled components back onto the chassis

5) Used some hot melt glue to hold the motor steady.

That's about the best I can get it. There is still some left/right thrashing of the rear axle caused by the now slight misalignment of the motor and driveshaft but with a flexible chassis, a spring-coupled driveshaft, and a driveshaft bearing having lots of slop, it is about the best it will ever be without upgrading components. It is a huge improvement on how it was out of the box. It's gone from being a coarse coffee grinder to a mild ratchety sound.

Thanks for all the suggestions.

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EdGasket

Discussion Starter · #16 · May 16, 2021

Oh forgot to mention that having bent the chassis back flat, the magnet is not so close to the track and there is a lot less friction. Hence the car is faster on the straights and the motor no longer gets hot due to excess friction.

Reactions: 1

Abarth Mike

· #17 · May 16, 2021

On a Scalextric Caterham with a bent spring I reversed the spring so that the bent portion was on the shaft and not in between.

Found this for you. http://www.homeracingworld.com/snakecharmer.htm it is so old that there is no mention of Paul Gage tires.

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EdGasket

Discussion Starter · #18 · May 16, 2021

Yes I'd seen that though I do not have the ultra-fast red-can motor; just a silver coloured one which is fine.

beardy56

· #19 · May 16, 2021

If you go to the home page and scroll down to the Aston Martin there is an alternative to the spring for front motors.

Abarth Mike

· #20 · May 17, 2021

You can also use the ink tube from a BIC ballpoint. Avant slot have a shaft/bearing/spring kit part # AS20557 if you can find one.

Avant_Slot_20557_lange_Motorwelle_fuer_F

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